In air conditioning systems for the aeration and conditioning of buildings or vehicles, the air generally not only has to be cooled, but also dehumidified since the air to be cooled often has such a high humidity that, upon cooling to the desired temperature, the dew point is fallen below. Hence in conventional air conditioning systems dehumidification of the air accounts for a large part of the electricity consumption.
One option of reducing the electricity consumption of air conditioning systems for buildings is the dehumidification of air by adsorption or absorption of water using a drying medium and a regeneration of the drying medium laden with water by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid adsorbent, the advantage of absorption in a liquid absorption medium is that drying of air can be performed with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium using solar heat is easier to carry out.
The aqueous solutions of lithium bromide, lithium chloride or calcium chloride hitherto employed as liquid absorption media in commercial air conditioning systems have the disadvantage that they are corrosive towards the metallic materials of construction typically employed in air conditioning systems and that they thus necessitate the use of expensive specific materials of construction. These solutions can additionally cause problems due to salt crystallizing out of the absorption medium.
Ionic liquids comprising dialkylimidazolium ions (as described in WO 2004/016631 A1) have been described as alternatives to lithium salts in the prior art for similar applications. Y. Luo et al., Appl. Thermal Eng. 31 (2011) 2772-2777 proposes the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of aqueous solutions of lithium bromide for drying of air.
Y. Luo et al., Solar Energy 86 (2012) 2718-2724 proposes the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for drying of air.
US 2011/0247494 A1 proposes, in paragraph [0145], the use of trimethylammonium acetate or 1-ethyl-3-methylimidazolium acetate as liquid drying agent instead of aqueous lithium chloride solution. Example 3 compares water uptake from humid air for a series of further ionic liquids.
However, a problem of ionic liquids comprising dialkylimidazolium ions is that they often comprise impurities, which lead to substances that are odour-intensive or are injurious to health entering the dehumidified air upon a dehumidification of air using the ionic liquid. Moreover, it has been found that during the desorption of water from ionic liquids which contain a basic anion, such as, for example, a carboxylate ion, odour-intensive decomposition products are formed which, in the event of a subsequent use of the ionic liquid for the dehumidification of air, enter the dehumidified air.
Therefore, there remains a need for ionic liquids comprising imidazolium ions, which do not display the disadvantages described above. The problem to be solved by the present invention is hence provision of a process for the production of ionic liquids comprising dialkylimidazolium ions, wherein the level of volatile compounds is brought to a minimum.